This invention relates generally to the production of chlorine dioxide in a concentration suitable for use as a disinfectant in wastewater treatment to destroy pathogens, and more particularly to an improved chlorine dioxide generating system which operates efficiently and economically to yield a chlorine dioxide output having a low percentage of chlorine.
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, flow then taking place into a tank or through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. The amount of chlorine added to the water is referred to as the "dosage," and is usually expressed as milligrams per liter (mg/l) or parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic, compounds and some inorganic substances, such as iron or manganese, is designated as the "demand."
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. The hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions as a function of the pH of the water. In either the hypochlorous or hypochlorite form, chlorine is called "free chlorine residual." Hypochlorous acid has a highly effective killing power toward bacteria, whereas the hypochloride ion is a much less effective disinfectant.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, called chloramines, are created. Chloramines may occur almost instantaneously, depending mainly on water pH.
Many applications exist for chlorine in wastewater treatment facilities, such as for odor control of raw sewage and the control of hydrogen sulfide in sewers, but its most universal application lies in wastewater treatment facilities for the terminal disinfection of the treated plant effluent just before the effluent is discharged. The term wastewater as used herein is intended to include all waters in need of treatment such as water derived from industrial and municipal discharges, as well as naturally occurring waters and agricultural runoffs.
The virtues of chlorination have long been appreciated, but it is only recently that the hazards involved in excessive chlorination have been publicly recognized. In studies carried out in the chlorinated water supply of the City of New Orleans, it was found that the levels of chlorination were such as to release carcinogenic agents dangerous to the community. The results of this study are reported in the article by R. A. Harris, "The Implication of Cancer Causing Substances in Mississippi River Water," published by the Environmental Defense Fund, Washington, D.C., Nov. 6, 1974.
Shortly after this study appeared, Public Law 93-523 went into effect authorizing the EPA administrator to conduct a comprehensive study of public water supplies "to determine the nature, extent, source of, and means of control of contamination by chemicals or other substances suspected of being carcinogens."
Subsequently, Jolly ("Chlorine-containing Organic Constituents in Chlorinated Effluents"--Journal of the Water Pollution Control Fed., 47:601-618 (1975) reported the presence of forty-four chloro-organic compounds in a chlorinated secondary wastewater effluent.
The formation of compounds suspected of being carcinogenic as a result of the reaction of chlorine with hydrocarbons in wastewater is by no means the only unwanted side effect caused by the traditional disinfection process, for chlorine residuals in wastewater give rise to an environment that is toxic to aquatic organisms. Though chlorine is a highly effective biocide for undesirable organisms, it is also deadly to fish and other forms of aquatic life and therefore has a deleterious impact on fresh water, marine and estuary eco-systems.
It is now recognized that chlorine dioxide (ClO.sub.2) can provide significantly better results in many wastewater treatment systems where the use of chlorine has been proven to be hazardous or relatively ineffective. Chlorine dioxide possesses valuable bactericidal and viricidal properties and has an oxidizing capacity over twice that of chlorine. The disinfecting ability of chlorine depends on the hypochlorous acid it forms when dissolved in water, the higher the pH the lower the proportion of hypochlorous acid present. Hence chlorine disinfection decreases markedly as the pH rises. But the effectiveness of chlorine dioxide is about the same over the entire pH range, which renders it a far more effective disinfectant at higher pH values.
But from the standpoint of public safety, the most important characteristic of ClO.sub.2 is that it does not react with ammonia and most ammonium nitrogen compounds, and though it degrades phenols, it does so without producing offensive chlorophenols. Thus while in contaminated waters, large quantities of chlorine can be consumed before creating the residual needed for disinfection, the use of ClO.sub.2 in the same situation usually entails much smaller quantities to reach the desired residual, for ClO.sub.2 is very selective in its reactions with organics and is non-reactive with ammonia.
Another important practical application for chlorine dioxide is as a disinfectant for cooling water systems in power plants. Chlorine dioxide is more effective in inhibiting algae growth in such systems than chlorine. Since the growth of algae impairs the heat transfer characteristics of the system, the use of a disinfectant is essential.
One known method for generating chlorine dioxide is to react sodium chlorite and chlorinated water in the manner disclosed for example in U.S. Pat. No. 4,013,761 by Ward et al. assigned to Olin Corporation, this process being known commercially as the Dioxolin Process System. As indicated in a report (undated) published by Olin Corporation entitled "Treatment of Water Supplies with Chlorine Dioxide" excess chlorine must be added to improve the reaction yield. An excess of chlorine will bring about a full conversion of sodium chlorite to chlorine dioxide, but only under strict pH and chlorine concentration limitations. As a consequence, the output of the system is constituted by a high percentage of chlorine as well as chlorine dioxide. Thus with this chlorine dioxide generating system one does not obviate the known hazards incident to the use of chlorine in wastewater.
The use of excess chlorine can be avoided, as indicated in the Ward et al patent, by reacting aqueous sodium chlorite, sodium hypochlorite and a mineral acid to generate chlorine dioxide, the mineral acid containing sulfuric acid or hydrochloric acid. And while the yield of this system is high and is also chlorine free, the system is relatively expensive to operate because of the high cost of mineral acids and sodium hypochlorite.
Another known approach to generating chlorine dioxide involves an enrichment loop as disclosed in the U.S. Pat. No. 3,975,284 to Lambert in which a concentrated aqueous chlorine solution is reacted stoichiometrically with a sodium chlorite solution. In this arrangement, known commercially as the CIFEC system, use is made of a recirculation loop of aqueous chlorine solution under a plug-flow regime which prevents negative hydraulic gradient conditions that are detrimental to the process. Recirculation is accomplished by a special positive displacement pump which also produces the hydraulic power to operate a chlorinator injector. A rotameter assembly and diaphragm valve regulates the feed of make-up water into the loop. A sodium chlorite metering pump injects a specified concentration of this solution into a chlorine dioxide reactor where the reaction between HOCl and NaClO.sub.2 produces ClO.sub.2.
While the enrichment loop system for generating chlorine dioxide yields a chlorine dioxide output which has a low percentage of chlorine, the system is relatively complex and, by reason of the recirculating pump, is high in its energy consumption.